Vortex Wakes of Oscillating Circular Cylinders

The arrangements of vortices in the wake behind a circular cylinder making transverse sinusoidal oscillations in a uniform water flow were investigated experimentally at the Reynolds number range between 70 and 163. The dimensionless amplitudes were 0.40, 0.67, 1.43, 2.28 and 3.80, and the dimensionless oscillation frequencies varied from 0.05 to 0.30. Flow patterns at various oscillation frequencies were photographed by using both the Hydrogen-bubble method and the condensed-milk method for flow visualization. Periods and phases of the wake vortex formation were measured. The drag on a cylinder making a self-excited oscillatory motion was also measured by using the balance method. The main findings in the present investigation are as follows; 1) By vibrating the cylinder laterally, it is possible to control the wake frequency over a range of roughly 30% below and 20% above the natural shedding frequency. 2) The constant phase of vortex formation is maintained over the locked-in region. 3) The block structure of the wake appears at the ends of the locked-in region. 4) When the dimensionless amplitude of the cylinder is larger than the order of unity, the strength of the vortices is not equall for the two rows and each individual vortex of the stronger row consists of two smaller vortices having opposite rotations. 5) The predominant shedding frequency of such a distorted vortex wake agrees with the driving frequency. 6) The drag on the freely oscillating circular cylinder is increased when the shedding frequency is locked in to the natural frequency of the cylinder.